The recent availability of genome sequence and microarray technology is bringing about a profound expansion in our level of understanding of host-microbe interactions. The proposed study will utilize a pathosystem in which both the pathogen and host are well characterized and genetically tractable, and whose genomes are fully sequenced. Early host responses of Arabidopsis thaliana to the unique pathogen and natural genetic engineer Agrobacterium tumefaciens will be investigated at the level of gene expression, taking advantage of newly available, comprehensive oligonucleotide microarrays representing all of the approximately 26,000 known and predicted open reading frames for Arabidopsis. Transcriptional changes elicited in Arabidopsis by Agrobacterium will be compared to those elicited by two other, dissimilar bacteria (the nonpathogenic E. coil and typical plant pathogen, Pseudomonas syringae) to identify plant responses unique to Agrobacterium challenge. Bacterial gene(s) which, in turn, are required to elicit the host response, will be identified. The insights gained from such a study will i) contribute to the understanding of eukaryote-bacteria interactions; ii) contribute information useful for exploiting Agrobacterium the natural genetic engineer, whose host range has been expanded recently to include fungi, yeast, and even mammalian cells, but is not universal, and iii) provide a basis for comparison of host responses to phylogenetically proximal members of the alpha-2 proteobacteria, which include plant symbionts and mammalian pathogens.